The measurement of electrical signals on the leads or legs of an integrated circuit (IC) package, or the application of external electrical stimulation to the legs of an IC, has become increasingly problematic with the continued decrease in the size of the legs and the space therebetween, coupled with increasing numbers of legs on a package. The circuit probing techniques originated for use with test equipment such as voltmeters and oscilloscopes are simply impractical for use with surface mount components of half-millimeter or less lead pitch. Increasingly, the task of connecting logic analyzers, timing analyzers, emulators and automatic board testers to surface mounted integrated circuits is becoming mechanically difficult, less reliable and definitely more expensive.
As the size of the legs and their intervening space decreases, a number of problems are exacerbated. Because of the legs' smaller size, there is less contact surface, and what there is of it may be coated with a residue of solder resist or flux. Contact mechanisms that rely upon making contact with the outer face of the legs run the risk of poor contact either because of poor wiping action against the leg by the corresponding contact in the probe or because of insufficient contact pressure. Alignment is becoming increasingly critical, with the associated risk that the probe's contacts may short adjacent legs together. To ensure that the probe does not fall off inadvertently, some manufacturers are equipping their IC probes with grippers that engage the comers of the IC package. These grippers have to be replaceable because they are easily broken.
A further problem arises with conventional probes: They tend to have a bigger "footprint" than the IC's they probe, and as those IC's get smaller, the footprints do not scale down as fast in size as do those IC's. Thus, as the density of circuit boards increases so does the problem of interference between a probe for an IC and the components adjacent that IC.
What is needed is an entirely new approach for connecting to the leads of an IC. Consider wedging a strip of conductive material between adjacent legs of an IC. If one side of the material is backed by an insulator then the adjacent legs will not be shorted together. Let the wedge be flexible for movement away from one leg and toward the other, so that the wedge can adjust to slight variations in leg size or position. This allows the wedge to "follow" the gap between the legs as it is forced between them in a direction normal to the surface of the board carrying the IC. By wedging in the direction normal to the board all adjacent legs can receive a wedge at the same time. The tip of a wedge can be considerably narrower than the space between adjacent legs of the IC. This allows a group of wedges to initially interdigitate quickly with the legs of an IC, and then later begin to exert sideways pressure against the legs as the actual wedging action occurs. The wedging action is accompanied by motion and by compression of the wedge between the legs. This ensures good wiping action, and thus improves the electrical connection.
In a preferred arrangement the wedges engage the legs of the IC immediately proximate where the legs emerge from the package of the IC. At this location the legs are essentially coplanar flat strips lying in a plane parallel to and somewhat separated from the surface of the board that is to receive the IC. This is a good location, since the case provides good support for holding the legs in position, and because at this location the spacing between the legs is already closely controlled during manufacture of the IC. The regularity of spacing arises from the precision step and repeat operations undertaken to construct the die used to remove the "dam bar" that initially joins the legs during injection molding of the IC package. (The dam bar joins and aligns the legs during manufacturing operations like wire bonding to the chip, making them into a unit that is easier to handle. It also serves to prevent the plastic from extruding from between those legs during the injection molding of the package. Once the package is molded, however, the dam bar needs to be removed, since it shorts all the legs together.)
The inner edge of the wedge which comes closest to the package of the IC as the wedge penetrates between the legs can be relieved at the tapered end of the wedge. This is the lower inside corner of the wedge, as it were; the relief can be accomplished either as a radius or as a shallow triangle. The purpose of this relief is to allow the probe to fit slightly loosely over the IC as the probe is started. This will occur because the distance between opposing wedges (i.e., wedges on opposite sides but the same distance away from a common intervening side) is slightly greater at their tips than further up at where the wedging actually occurs.
It will also be appreciated that the wedges could, if so desired, be disposed so that they interdigitate with that portion of the legs that traverse the height difference between where the legs exit the package and where they are soldered to the circuit board. That is, on the (possibly straight) intervening portion between the opposing bends of the S-shape given to the legs.
Furthermore, there need be no limitation on the concept of wedging that limits it to use with high density surface mount IC's. Suitable wedges for use with older styles of IC packaging are quite feasible. Nor must it be the case that the IC be soldered to a circuit board; the shape of the wedges can be selected to function when the IC they are probing is installed in a socket that is itself mounted on a printed circuit board.
The interdigitation relied upon for producing wiping action and contact pressure assures that there will be no shorting between adjacent IC pins or legs. This is because the length and width of the conductive surface of the wedge are each at right angles to the direction in which the IC legs occur in sequence. The interdigitation also assists, perhaps in conjunction with guides engaging the corners of the IC package, in automatic self alignment of the probe with the IC.
In principle, the wedges need not attach to any structure or support larger than or located outside the outline (footprint) of the IC itself (which includes its legs). That is, the outermost extent of a probe incorporating wedges (the probe's extended footprint, as it were) can be less than, and located entirely within, the footprint of the IC. The wedges themselves can be attached to a supporting structure within the probe that is very comfortably on the inside of the footprint. This means that interference between the probe and parts closely adjacent to the IC can be avoided.
IC's with perimeter leads generally have two or four rows of legs, or at most one row along each side of the package. If a row has n legs then n wedges conductive on just one side are sufficient. With n wedges the wedge at one end of the row is in a different situation than the others, since it does not wedge between two adjacent legs. Instead, it must rely upon its own rigidity to produce sufficient contact pressure with its corresponding leg.
Further advantages may be obtained by making the wedges conductive on two sides (that are separated by an intervening insulator) and by increasing the number of wedges to n+1 so that the n intervening spaces therebetween align with the n legs. Adjacent wedges have conductive portions that face each other; these portions are electrically connected together. First, this doubles the amount of electrical contact on each leg. Second, it makes the aggregate force applied to each row of legs symmetrical, since now each leg at each end of a row is contacted by an inner-most portion of each outer-most wedge.
The individual wedges themselves may be fabricated by laminating gold plated copper foil with one or more layers of a suitable thin plastic and an adhesive. Wedges to be conductive on both sides have copper foil on both sides. The tapered portion at the tip of a wedge may be formed by giving different lengths to the interior layers of thin plastic and adhesive. Alternatively, the tip of a double sided wedge may be etched to remove some of the material separating the two outer conductive foils. A plurality of wedges may be laminated together with adhesive and plastic spacers to form rows of wedges that will interdigitate with the legs of the IC. The rows of wedges are bonded to a carrier or header, and electrical connections are made between the copper foil and the interconnecting wires or cables that are attached at their other end to whatever measurement device or test equipment that uses the probe. The result is a relatively inexpensive probe that may be readily pressed onto the IC of interest, does not interfere with surrounding components, makes good electrical contact, and that will reliably remain affixed to the IC through simple friction until removed.